1. Field of the Invention
The present invention relates to a cold cathode tube lighting device, tube current detecting circuit to be used in a plurality of cold cathode tubes, tube current controlling method, and integrated circuit and more particularly to the cold cathode tube lighting device which can be suitably used when the plurality of the cold cathode tubes being used as a backlight for a liquid crystal display device is driven by pulses output from inverters supplied to input terminals on both sides of each of the cold cathode tubes, to the tube current detecting circuit to be used in the cold cathode tube lighting device, the tube current controlling method, and the integrated circuit.
The present application claims priority of Japanese Patent Application No. 2005-241682 filed on Aug. 23, 2005, which is hereby incorporated by reference.
2. Description of the Related Art
In recent years, a liquid crystal display device is used not only for monitors of personal computers but also for various display devices such as liquid crystal panel television sets. In the case of the liquid crystal panel television sets or a like in particular, upsizing of a liquid crystal panel itself is progressing. As a result, a backlight used in each of the liquid crystal display devices is increasing in size and a cold cathode tube in the backlight is also made long. When a cold cathode tube is to be lit, in the case of a short cold cathode tube, its one input terminal is used as a low-voltage side and the other input terminal as a high-voltage side and a driving pulse is input from an input terminal on the high-voltage side.
However, in the case of a long cold cathode tube or a cold cathode tube having a small diameter, since impedance of the cold cathode tube becomes high, when a driving pulse is input from one input terminal (on a high-voltage side) of the cold cathode tube, a display area in a region near to the input terminal on the high-voltage side becomes bright and the display area in a region near to the input terminal on the low-voltage side becomes dark, causing a luminance gradient to occur. To prevent the occurrence of the luminance gradient, a both-side high-voltage driving method is employed in which a cold cathode tube is made to light by applying driving pulse voltages with different phases to input terminals on both sides of the cold cathode tube. Moreover, in order to improve the efficiency of the backlight, even in the case where a cold cathode tube is of “U”-shaped or “”-shaped or even in the case where a diameter of the cold cathode tube is small, the both-side high-voltage driving device is used in some cases. Moreover, there is a method by which a plurality of cold cathode tubes is made to light by using one inverter. However, if the cold cathode tube is long, unless high voltages are input from input terminals on both sides of the cold cathode tube, a luminance gradient occurs in the cold cathode tube.
Luminance of a cold cathode tube is determined by a tube current flowing through the cold cathode tube. Therefore, in a one-side high voltage driving method in which driving pulses are applied to an input terminal on one side of a cold cathode tube, a current detecting circuit made up of resistors or a like is mounted on a low-voltage side to which driving pulses are not applied to exercise control to keep constant the luminance of the cold cathode tube based on detected current values, whereas, in the both-side high voltage driving method, voltages of the driving pulses which are applied to both the input terminals of the cold cathode tube are high and a current detecting circuit such as a resistor cannot be inserted, which, as a result, makes it impossible to detect a tube current of the cold cathode tube.
Conventional technology of this type is disclosed in following References. A driving device of a piezoelectric transformer disclosed in Patent Reference (Japanese Patent Application Laid-open No. 2002-017090, Abstract, FIG. 1), as shown in FIG. 12, includes a power source 11, a driving circuit 12, a variable oscillating circuit 13, an oscillation controlling circuit 14, a piezoelectric transformer 15, a voltage detecting circuit 16, a current detecting circuit 17, a phase difference detecting circuit 18, and an effective current detecting circuit 19. Between the piezoelectric transformer 15 and the current detecting circuit 17 is connected a cold cathode tube 20. A reflecting plate 21 being grounded is connected near the cold cathode tube 20 and floating capacitance Cx is formed between the cold cathode tube 20 and the reflecting plate 21. In a driving device of the piezoelectric transformer 15, a tube current (current output from the piezoelectric transformer 15) of the cold cathode tube 20 is detected by the current detecting circuit 17 and a phase difference between a current and voltage output from the piezoelectric transformer 15 is detected by the phase difference detecting circuit 18. Based on the detected phase difference, an effective current flowing through the cold cathode circuit 18 is detected by the effective current detecting circuit 19 and the piezoelectric transformer 15 is controlled for driving via the oscillation controlling circuit 14, variable oscillating circuit 13, and driving circuit 12 so that the effective current becomes equal to a predetermined set value.
In a discharge tube inverter circuit for lighting multiple lamps disclosed in Patent Reference 2 (Japanese Patent Application Laid-open No. 2004-335443, Abstract, FIG. 1), driving pulses are applied from one inverter through a shunt transformer to a plurality of discharge tubes to cause each of cold cathode tubes to be lit. The shunt transformer has inductance exceeding a negative resistance characteristic of the cold cathode tube. By adjusting the inductance, a tube current flowing through each cold cathode tube is made uniform.
In a cold cathode tube light-calibrating device disclosed in Patent Reference 3 (Japanese Utility Model Gazette 3096242, Abstract, FIG. 1), driving pulses fed from a high-voltage side of an inverter are supplied through a ballast capacitor to an input terminal on one side (high-voltage side) of each of two or more cold cathode tubes. On a low voltage side of the inverter is connected a current detecting circuit made up of a resistor and, based on a detected current value, a duty ratio of each of the driving pulses is controlled to exercise control to keep the luminance of the cold cathode tube constant.
In a separately-excited inverter disclosed in Patent Reference 4 (Japanese Patent Application Laid-open No. 2001-052891, Abstract, FIG. 1), there are provided an inverter transformer whose primary winding is of a push-pull configuration, two switching elements to control on/off both sides of the primary winding, and a clock signal generating circuit to supply clocking signals with different phases to the two switching elements. This allows oscillation frequency to be set freely without a constraint of a resonance frequency of the inverter transformer.
In the case of a discharge lamp lighting device disclosed in Patent Reference 5 (Japanese Patent Application Laid-open No. 2004-235123, Abstract, FIG. 1), in a video device using a cold cathode tube as a light source, driving pulses fed from a high-voltage side of an inverter are applied to an input terminal on one side (high-voltage side) of one cold cathode tube. On a low-voltage side of an inverter is mounted a current detecting circuit made up of resisters and, based on a detected current value, a tube current of the cold cathode tube is controlled by a PWM (Pulse Width Modulation) method and resolution obtained by the PWM method is expanded by a bit reducing circuit.
In a cold cathode tube lighting device disclosed in Patent Reference 6 (Japanese Patent Application Laid-open No. 2005-063941, Abstract, FIG. 1), a plurality of cold cathode tubes is lit in a uniform and stable manner by a low-impedance power source serving as one common power source and by a plurality of ballasts connected to at least one of two or more cold cathode tubes.
In the lamp driving circuit disclosed in Patent Reference 7 (Japanese Patent Application Laid-open No. 2005-063970, Abstract, FIG. 1), a temperature sensor is mounted near to an external electrode of a lamp and a state of the lamp is monitored. As a result of the monitoring, when a temperature of the lamp falls within a critical temperature range, a tube current decreases and when the temperature exceeds the critical temperature range, supply of power to the lamp is turned OFF.
However, the above conventional technologies have the following problems. In the driving device of the piezoelectric transformer disclosed in the Patent Reference 1, due to a high voltage output from the piezoelectric transformer 15, a high-withstand component is required as a component to which such a high voltage is applied, causing high costs. A tube current is detected on one side of the cold cathode tube 20 and, therefore, a current of the tube cannot be detected exactly due to variation between terminals in the piezoelectric transformer 15 and/or cold cathode tube 20.
In the discharge tube inverter circuit for lighting multiple lamps disclosed in the Patent Reference 2, though a tube current flowing though each cold cathode tube is made uniform, since a value of the tube current cannot be changed, no control to keep the luminance of the cold cathode tube constant is exercised. The purpose of the cold cathode tube light-calibrating device disclosed in the Patent Reference 3 is to drive each cold cathode tube by one-side voltage driving method and, therefore, the purpose and configuration of the conventional device are different from those of the present invention.
The purpose of the separately-excited inverter disclosed in the Patent Reference 4 is to perform light calibration independently on a plurality of cold cathode tubes and, therefore, the purpose and configuration of the conventional inverter are different from those of the present invention.
In the case of the discharge lamp lighting device disclosed in the Patent Reference 5, the cold cathode tube is driven by the one-side high voltage driving method and, therefore, the purpose and configuration of the conventional inverter are different from those of the present invention.
In the cold cathode tube lighting device disclosed in the Patent Reference 6, a plurality of cold cathode tubes is driven by the one-side high voltage driving method and the purpose and configuration of the conventional inverter are different from those of the present invention.
In the lamp driving device disclosed in the Patent Reference 7, a temperature of the lamp is detected by the temperature sensor and the supply of power to the lamp is controlled and, therefore, the purpose and configuration of the conventional inverter are different from those of the present invention.